The ability of seminal plasma to influence the fertility of ejaculated bull spermatozoa was examined using a sperm penetration assay for zona-free bovine oocytes. Washed, ejaculated spermatozoa from bulls of below (low) or above average (high) fertility were mixed with seminal plasma from the same bull, or with seminal plasma from a bull of contrasting fertility. Treated spermatozoa were stained with different fluorochromes and competed to penetrate oocytes after heterospermic insemination in vitro. Washed spermatozoa exposed to seminal plasma from bulls of high fertility penetrated more oocytes than those spermatozoa mixed with seminal plasma from bulls of low fertility (P < 0.01). Mixing low fertility spermatozoa with high fertility seminal plasma generally improved penetrating ability compared with low fertility spermatozoa mixed with low fertility seminal plasma (P = 0.05). Washed spermatozoa from a bull of low fertility mixed with his own seminal plasma had greater ability to penetrate oocytes than did washed spermatozoa from a bull of high fertility mixed with seminal plasma from a bull of low fertility (P < 0.01). The bias associated with using low fertility seminal plasma from the bull providing the spermatozoa was removed by repeating this experiment using pooled seminal plasma from different subfertile bulls. After combination with pooled seminal plasma from bulls of low fertility spermatozoa from bulls of high or low fertility penetrated oocytes in a similar way, but high fertility spermatozoa had a slightly higher penetration rate (P = 0.12). In conclusion, the penetration of zona-free oocytes by ejaculated spermatozoa from bulls of low fertility was marginally improved by seminal plasma from bulls of high fertility, but penetration by high fertility spermaotoza was decreased by exposure to low fertility seminal plasma. Seminal plasma from bulls of low fertility similarly affected the penetrating ability of high and low fertility spermatozoa if the seminal plasma used was foreign to the spermatozoa being tested.

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